Microelectrodes that can be implanted for a long time into the central nervous system (CNS) have a wide field of application. In this invention, the term “electrode” refers to a microelectrode. In principle, all brain nuclei can be recorded from or stimulated by such electrodes and their functions monitored. Of particular importance is the use of a multichannel design in brain nuclei stimulation. In such a design groups of electrodes or even individual electrodes can be addressed separately. This allows the user to select those electrodes whose stimulation produces a therapeutic effect that is improved in comparison with unselective stimulation. Stimulation of the brain or spinal cord can be of particular value in situations when brain nuclei are degenerated or injured. In certain situations it would also be useful to be able to combine controlled electrical stimulation and local gene transfer. A multichannel design may also allow the user to effectively measure the effects on multiple neurones and other cells following systemic or local drug administration or gene transfer. Of particular interest is an ability to simultaneously measure the effects of multiple drug candidates on neuronal function. Monitoring brain activity through implanted electrodes can also be useful if used to control drug delivery either locally or systemically or other therapeutic methods such as electrical stimulation of brain nuclei. Multichannel electrodes may also be used to lesion specific and circumscribed sites in tissue after abnormal impulse activity has been detected by recordings from the electrodes or by imaging such as fMRI or PET.
To record and stimulate brain structures various forms of implantable electrodes have been developed (U.S. Pat. Nos. 6,253,110 B1, 5,957,958, 4,573,481, 7,146,221 B2, 5,741,319, 4,920,979, 5,215,008, 5,031,621, 6,993,392 B2, 6,032,062, 4,852,573, 3,995,560, 7,041,492, 6,421,566 B1, 4,379,462, 5,417,719, 3,822,708, 5,501,703, 7,099,718 B1, 3,724,467; US 2007/0197892 A1). However, little attention has been paid to the injuries and complications caused by the implantation procedure. Not only can these consequences lead to an impaired function of the implant, they may also harm the individual in which the electrodes are implanted. The function of the implanted electrodes and also of the tissue, in which the implant is introduced, may be impaired due to either 1) acute injury of the tissue including bleeding and infarction of the tissue, 2) infection, 3) tissue reactions including inflammation and glial activation caused by the implantation procedure, 4) long lasting tissue reaction including glial activation and scar formation isolating the implant, and/or 5) movements between electrode and tissue. These consequences usually occur during different time intervals during and after the implantation:
1) When implanting microelectrodes in central nervous tissue there is, besides the general risk of open surgery, local risks such as bleedings and also infarctions of the tissue. Electrodes may punctuate blood vessels during implantation. This may cause bleeding and vasoconstriction. A strong inflammatory response to blood cells and proteins that leaked into the neural tissue can be thus triggered and might affect the tissue over extended periods of time. This may in turn induce cell death in the area supplied by the affected vessel. As a consequence the function of the electrode implant can be impaired.
2) General surgery and particular implantation of artificial devices also increase the risk of infections. The surgical area may be infected at the time of surgery or within the early recovery phase after surgery. The presence of a foreign body material (the implant itself) can also function as a locus minoris for establishing an infection in the tissue surrounding the electrodes. Tissue infections around implants are in general more difficult to treat with antibiotics than other tissue infections. Infections close to the implanted electrodes may besides impairing the function of the tissue also jeopardize the function of the implanted electrodes and may in extreme cases require removal of the device in order to cure the infection.
In addition to the general protection of systematically administered antibiotics it would be advantageous to be able to treat infections close to the implanted electrodes locally.
3) Implantation of electrodes into central nervous tissue will due to the inflicted injury always cause an acute inflammation (Ghirnikar, R S et al., Neurochemical Research 1998, 23(3):329-340; Norton, W T, Neurochemical Research 1999, 24(2): 213-218). This is a normal physiological reaction and is necessary for the healing process. In the case of a permanently anchored material in the tissue, the foreign body material may in addition induce a chronic inflammation that in the worst cases may jeopardize the function of the implant and the tissue. The material and the procedures related to the devices should therefore minimize chronic inflammation. Another complication that needs to be addressed is the reaction of the glial cells, particularly the astrocytes and microglia. In the event of perforating injuries to the central nervous tissue the astrocytes will proliferate and form an astroglial scar (Eng, F E et al., Neurochemical Research 2000, 25: 1439-1451; Polikov, V S et al., 2005, J Neuroscience Methods 148; 1-18)). Such a scar may form a capsule-like structure surrounding an implanted electrode and thereby insulate it from the rest of the central nervous tissue. In cases of a large astroglial capsule this may impair the function of the electrodes. Thus, the astroglial reaction needs to be controlled. It should not be totally prevented, however, since there are indications that lack of astrocytic involvement will actually cause a widespread inflammation and tissue reaction worsening the scenario (Eng, F E et al., Neurochemical Research 2000, 25: 1439-1451; Sofroniew, M V et al., Neuroscientist 2005, 11(5): 400-407). Microglia may also proliferate after an implantation. These cells have phagocytic capacity and they also release a number of substances that can trigger a chronic type of inflammation. By controlling the microglia the formation of the astrocytic capsule may be reduced. Besides being a health risk, the tissue reactions caused by different types of glia cells may impair the function of the implanted electrodes. For example, in case a zone devoid of neurons is created around the electrodes, or a scar is established, much higher current will be needed to activate living neurons. The increased current necessary to stimulate neurons at a distance may in turn cause further tissue damage through heat dissipation and/or through induction of irreversible reduction and oxidation reactions (see also U.S. Pat. No. 6,316,018)
4) The design of the multichannel electrode itself may also trigger delayed and long lasting tissue responses after implantation at least partly due to movements between electrode implant and the tissue. Of particular importance is the endogenous movements caused by breathing and by the heart beat. The consequent pulsatile movements are usually not uniform in the tissue. For example, the movement caused by the heart beat around a major artery propagates through the tissue in a nonuniform way. Movements between implant and tissue occurs if the implant is rigid or attached to a rigid structure that do not move along with the soft tissue in which the electrodes are implanted. It would therefore be an advantage to use flexible electrodes that after implantation are anchored in the tissue rather than in the skull or skeleton and that can follow the movements of the tissue and thereby avoid the friction between electrodes and tissue that otherwise may occur and which may trigger tissue responses. Movements between electrode and tissue can cause an unstable recording/stimulation situation and thereby impaired function of the implant. To reduce the movement between individual electrodes and the adjacent tissue caused by such endogenous movements, the electrodes should therefore be able to follow the tissue movements in all directions.
A further complication with multichannel electrodes used for research, in particular multichannel electrodes composed of numerous electrodes is that it is usually difficult or impossible to identify the neurones or cells recorded/stimulated by the individual electrodes. This hampers the interpretation of the results considerably since it is not possible to relate the recorded signals to cell type or cell morphology. This also may cause problems in interpretation of the effects of stimulation since it is not clear which cells were stimulated.
It would thus be a substantial advantage if the aforementioned complications could be avoided or at least alleviated.